Destructive Tree Diseases Associated with Ambrosia and Bark Beetles: Black Swan Events in Tree Pathology?

Abstract

Diseases that are associated with ambrosia and bark beetles comprise some of the most significant problems that have emerged on trees in the last century. They are caused by fungi in the Ophiostomatales, Microascales and Hypocreales, and have vectors in the Scolytinae (ambrosia and bark beetles), Platypodinae (ambrosia beetles) and Hylesininae (bark beetles) subfamilies of the Curculionidae (Coleoptera) (73,102,144). Some of these problems, such as Dutch elm disease (DED), have a long history, have been extensively researched, and are fairly well understood (23,56,70,113,149,168,169). In contrast, other similar diseases developed recently and are poorly or partially understood (2,3,42,51,73,89,94,98,114,127,137,154,158, 164,165). Significant data gaps may exist for the ecology, epidemiology and management of the latter diseases.The emergence and unexpected importance of these tree diseases are discussed in thisarticle. An underlying factor in most of these interactions is the absence of a coevolved history between the so-called "naïve" or "new encounter" host trees and the pathogens and/or beetles (27,128,174). For the ambrosia beetles, these interactions are associated with susceptibility to what are typically benign fungi and atypical relationships with healthy trees (ambrosia beetles favor trees that are dead or stressed). Interestingly, the pathogens for both the ambrosia and bark beetle-associated diseases often have symbiotic relationships with the insects that are not based on phytopathogenicity. Some of the most alarming and damaging of these diseases are considered below as "black swan events" (155).Black Swans. Before 1697, improbable events and situations in Europe were known as "black swans" (at the time, all swans known to Europeans were white) (134). In that year, the black swan, Cygnus atratus, was discovered in Western Australia (38). Thereafter, "black swan" developed as a metaphor for a supposed impossibility that is contradicted with new information.For example, John Stuart Mill used black swan logical fallacy when identifying falsification, a key component in the scientific method (68).In a recent book, Taleb (155) developed Black Swan Theory (BST). Unlike the "black swan" to which Mill referred (68), BST focuses on unexpected events of large magnitude and consequence (155). Taleb (155) recognized such events in diverse fields including finance, history, science and technology. He suggested that black swan events: 1) have extreme impacts; 2) lie outside the realm of regular expectations (they are rare); and 3) are unpredictable.Although all ambrosia and bark beetle-associated diseases are not black swan events, several do fulfill the above criteria since they have large impacts, are uncommon surprises and are unpredictable (2,3,51,56,73,89,94,98,114,137,154,158, 164,165) (Table 1). They are typically understood and appreciated only with the benefit of hindsight and subsequent research.The Beetles and Beetle-associated Tree Diseases. The order Coleoptera contains more species than any other or...

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